Fig. 359.
Section of nucellus and endosperm of white pine. The inner layer of cells of the integument shown just outside of nucellus; arch, archegonium; en, egg nucleus. In the nucellar cap are shown three pollen tubes. vn, vegetative nucleus or tube nucleus; stc, stalk cell; spn, sperm nuclei, the larger one in advance is the one which unites with the egg nucleus. The archegonia are in the endosperm or female gametophyte. (After Ferguson.)
622. The endosperm.—In the following spring growth of all these parts continues. The nuclei in the macrospore divide to form more, and eventually cell walls are formed between them making a distinct tissue, known as the endosperm. This endosperm continues to grow until a large part of the nucellus is consumed for food.
Fig. 360.
Last division of the egg in the white pine cutting off the ventral canal cell at the apex of the archegonium. End, endosperm; Arch, archegonium.
623. Female prothallium and archegonia.—The endosperm is the female prothallium. This is very evident from the fact that several archegonia are developed in it usually on the side toward the pollen chamber. The archegonia are sexual organs, and since the sexual organs are developed on the gametophyte, therefore, the endosperm is the female gametophyte, or prothallium. In [fig. 359] are represented two archegonia in the endosperm and the pollen tubes are growing down through the nucellus. The archegonia are quite large, the wall is a sheath or jacket of cells which encloses the very large egg which has a large nucleus in the center.
624. Pollen tube and sperm cells.—While the endosperm (female prothallium) and archegonia are developing the pollen tube continues its growth down through the nucellar cap, as shown in [fig. 359]. At the same time the two cells which were formed in the pollen grain (antheridium) from the central cell move down into the tube. One of these is the “generative” cell, or “body” cell, and the other is called the stalk cell, though it is more properly a sterile half of the central cell. The nucleus of the generative cell, about the time the archegonium is mature, divides to form two nuclei, which are the sperm nuclei, and the one in advance is the larger, though it is much smaller than the egg nucleus.
625. Fertilization.—Very soon after the archegonia are mature (early in June in the northern United States) the pollen tube grows through into the archegonium and empties the two sperm nuclei, the vegetative nucleus and the stalk cell, into the protoplasm of the large egg. The larger of the two sperm nuclei at once comes in contact with the very large egg nucleus and sinks down into a depression of the same, as shown in [fig. 361]. These two nuclei, in the pines, do not fuse into a resting nucleus, but at once organize the nuclear figure for the first division of the embryo. Two nuclei are thus formed, and these divide to form four nuclei which sink to the bottom of the archegonium and there organize the embryo which pushes its way into the endosperm from which it derives its food ([fig. 362]).
